Various manufacturing approaches are used for constructing structures in which the structure material is heterogeneous and consists of two basic types of materials, both in terms of shape and composition. One material type is in the form of high-strength high-elasticity-modulus filaments arranged substantially orthogonally so as to provide enhanced strength and stiffness in three basic or principal stress directions. The other material is used to interconnect the reinforcing filaments by means of surface bonding so as to transmit and distribute stress loadings between filaments along those three principal directions, and forms a matrix in which the reinforcements are embedded.
It is very difficult to fabricate thick sections of such reinforced material in which the reinforcing filaments and/or fibers are maintained straight throughout their lengths, in which the volume of matrix as percentage of the total volume is small and in which all three reinforcements are consistently in close proximity throughout the structure. A method for making such materials is described in my U.S. Pat. No. 3,577,294 entitled: METHOD FOR MAKING 3-D FILAMENT REINFORCED ARTICLES, but its application is limited to relatively thin shell sections. This limitation is imposed by the manner in which the filament winding is performed. A mandrel having an external surface covered with thin stiff bristles is used as a form upon which helically-wound filaments are laid, i.e. between the projecting bristles. The section thickness is thus limited by the bristle length. The latter is limited in turn by the risk that projecting bristles will be bent or broken by the filament being helically wound prior to being laid on the form. This risk augments compoundingly when the bristles are lengthened, because: (1) the degree of bending solicitation is higher when a bristle projects out farther, and (2 ) concomitantly, the resistance to bending of a bristle is highly reduced by an increase of its length.
The two compounding difficulties listed above are further increased if and when helical filament winding is to be performed about a form which exhibits a saddle-shape double curvature as is the case for rocket engine nozzles in the throat region. This becomes very apparent later in this disclosure. In addition, especially in the case of thick-shell structures, the proportion of radial to helically-wound reinforcing requirements may vary widely between locations near the internal surface and close to the external surface. It is practically impossible to adjust the nature and cross-section size of a continuous bristle along its length. Also, significantly increasing the diameter of individual radial reinforcing elements further facilitates the helical winding of thicker filament strands, which may then be programmable, wound so as to minimize the degree of physical interference earlier mentioned. The goals of the present invention are to provide a method and an apparatus which offer such possibilities, by using relatively short stubby spikes, instead of thin bristles, which are added to the structure while it is being constructed, at a required rate.